The present invention relates to a measuring sensor, comprising at least two rigid detector legs movable to varying distances from each other; detector elements and possible emission elements on one or both legs of the measuring sensor; pivoting means, the detector legs being tiltable relative to each other around a pivot axis line established thereby; at least two rigid operating legs, each being attached to one detector leg and, as an extension thereof, extending therefrom beyond the pivot axis line; and a spring element between the operating legs for producing a compressive force between the detector legs containing a piece of living tissue having one or a few of tissue dependent characteristics measured in a non-invasive manner by means of the measuring sensor, and said spring element being composed of an elastomer material. The invention relates also to the use of such a measuring sensor.
A typical application for the above-described type apparatus is currently a pulseoximetry sensor, but it is also possible to apply a similar type of measuring sensor for investigating or measuring some other characteristic of a living tissue, whereby the detectors and possible emitting sources must of course be chosen in consistency with the characteristic to be measured. Pulseoximeters are used e.g. for measuring the blood oxygen saturation of a patient non-invasively and continuously. The term non-invasive refers to the fact that no physical means are used to penetrate under the patient's skin, but the measurement is carried out by means of radiation and, aside from that, the action takes place externally of the body. Such monitoring of blood oxygen saturation is presently quite commonplace and one of the monitoring parameters required in many conditions. The measurement is optical and based on different absorption characteristics of red and infrared light in blood hemoglobin. At present, the pulseoximetry sensors have normally two light sources, typically radiation emitting diodes (LED), and one detector, said radiation sources being operated alternately in a synchronized fashion. In either case, the radiation sources and detectors can be first of all included in the sensor itself in the proximity of an object to be measured and from the sensor extend electrical cables to a surveillance monitor. However, the above-described conventional solution cannot be used, for example, during the operation of more and more popular magnetic imaging of a patient or some other powerful electromagnetic source. In these conditions, for example the electrical cable of a sensor would function as an antenna and all metal components in the measuring sensor or, generally defined, the components affecting a magnetic field and/or electrically conducting components interfere with the magnetic imaging apparatus and distort the imaging result, i.e. all such components in a measuring sensor cause trouble and ruin the magnetic images. Thus, the pulseoximetry sensors applicable in a magnetic imaging environment are typically designed by using fiberoptics. Hence, the light sources and the detector or detectors are located remotely from the actual area of imaging, such as on the housing of a surveillance monitor or in some other corresponding location. The light power is carried to and from a measuring location by means of a fiberoptic cable, comprising in practice a bundle of fibers constituted by a multitude of thin optical fibers and having a diameter which is typically 1-3 mm.
The above type measuring sensors, used for example in pulseoximetry, are generally composed of rigid detector legs and operating legs as extensions thereof, the detector legs and operating legs having a junction which is provided with pivoting means, such as a sort of link mechanisms or the like, whereby the operating legs can be pressed for spreading the detector legs from each other, the piece of living tissue to be measured, such as a finger, a toe, or an ear, being thus insertable between the detector legs. In addition to this, the measuring sensor must include some type of a spring element for maintaining the detector legs pressed against a piece of living tissue during a measuring process. The publication EP-0,716,830 A1 discloses a measuring sensor which is only provided with detector legs, but not with operating legs, and which mentions a spring element integral with the structure, the publication containing, however, no specific description in terms of the material, function, design, or other details of such element. The publication U.S. Pat. No. 5,279,295 describes one of the most common measuring sensor designs, wherein the detector legs can be pressed against a member of living tissue, such as a finger, by means of a rubber band extending around the detector legs. This is a very awkward solution and requires in a practical situation the fitter of a measuring sensor to have nimble fingers and to perform a considerable amount of all sorts of nibbling. Moreover, such a rubber band is an item that comes off and is lost easily, rendering the measuring sensor useless. The publications WO 92/21281 and WO 96/00518 introduce measuring sensors which include not only detector legs but also at least some kind of operating legs for pivoting the detector legs around a physical axle or a virtual axis. These measuring sensors employ metal springs, especially a clip type of spring, for producing a compressive force between the measuring sensor and a finger serving as the object to be measured. As pointed out above, all metal components present in a measuring sensor interfere at least in an environment of magnetic imaging with the production of a satisfactory magnetic image. In many other applications as well, it is advisable that the measuring sensor not be provided at least with any considerable numbers of metal components.
In addition to the above-described solutions, a company called HOK INSTRUMENT AB has introduced to the market a pulseoximetry sensor entitled "SafeSAT". The sensor is intended to be attached to a finger for measuring the oxygen saturation of blood. This measuring sensor is provided with two detector legs and, as an extension thereof, rigidly connected operating legs, the junction area of said leg members being provided with a simple mechanical hinge axle. As for the detector legs, the structure is relatively conventional, comprising soft pads coming into contact with a finger and radiation emitting components in one leg and detecting components in the other leg, which are by way of optical cables in communication with actual detectors and radiation sources located externally of the measuring sensor. The above-discussed spring force is established by means of two lengths cut off an elastic tube of silicone rubber and fitted between the operating legs, the axial lines of said tubes being parallel to the detector legs and to said pivoting axis of the operating legs. Hence, these lengths of silicone tube are perpendicular to the moving direction of the detector legs. The operating legs are provided with inner surfaces facing each other, which are completely smooth and flat and the lengths of silicone rubber tube are bonded securely to these surfaces. The lengths of silicone rubber tube are by no means attached to each other, but the immobilization thereof is only achieved by the bonding between the silicone rubber tube and the inner surface of the operating legs. The bonding between elastic silicone rubber and some other type of plastics is very difficult to achieve and requires a plurality of working steps, in spite of which the bonding result is always quite unreliable. Thus, many of these sensors become useless even during the first time of use, in other words, really, it cannot be used used even for the first time as the bonding between a silicone rubber tube and an operating leg unfastens, the compression between the detector legs and the finger becoming unreliable or failing completely. Furthermore, in this marketed solution, the optical cables are brought out of the measuring sensor through cut-outs in the silicone rubber tubes, the making of said cut-outs requiring manual work or at least a separate working phase. This also adds to the costs of this solution. Yet another drawback in this marketed solution is the large number of various cut-outs, recesses, and the like, which are required by silicone rubber tubes and optical cables running therethrough and which easily accumulate dirt. Thus, the hygiene, or rather sterility, of this measuring sensor marketed by HOK INSTRUMENT AB is highly questionable, particularly in continuous use. In fact, it can be argued that, for example in surgical conditions, such a non-hygienic or poorly sterilizable instrument should not be used at all.
The publication U.S. Pat. No. 5,611,337 describes a pulseoximetry sensor attachable to an ear lobe, comprising a one-piece injection molded support structure provided with two legs set on the opposite sides of an ear lobe. One leg is provided with a measuring assembly consisting of a radiation source and a detector. The described structure includes a web, which is provided with a rib projecting therefrom in the direction of a plane parallel to the legs and arranged either on the web surface facing the legs or on the web surface facing away from the legs. According to the cited publication, the legs press against the ear lobe as a result of the elasticity of this rib projecting from the web. Admittedly, this described construction is smooth in terms of its outer surfaces and, thus, the cleanliness and sterility of such a pulseoximetry sensor are readily sustainable. However, the construction has also drawbacks. First of all, since the inter-leg compressive force is produced by means of a rib extending in the moving direction of the legs, which in one embodiment constitutes a spring based solely on tensile stress or in another embodiment a spring based solely on compression stress, the inter-leg force depends very definitely on a distance between the legs. A consequence of this is that the thickness of a measured object must always be quite accurately the same, since an object thinner than usual results in a very weak compression and, thus, in a risk of the entire sensor falling off, while an object thicker than usual results in an excessively strong compression and, thus, at least in discomfort for a patient but possibly also in a reduced blood circulation in the area of measurement and possibly thereby in an incorrect measuring result. This sensitivity to excessively weal and excessively strong compression is further enhanced by the shortness of the legs. In the depicted structure, the legs cannot be expanded to surpass the length or width of the measuring assembly, since this results in the entire construction becoming far too floppy and inexact, the measuring elements being able to shift into uncontrollable positions. In addition, the sensor legs have such a surface contour that under no circumstances is the depicted device able to hold its position on top of a narrow and convex-shaped measuring object, such as a finger or a toe, but will be slipping off. Thus, the measuring sensor shown in the cited publication does not have a very good range of utility.